Many centuries ago, the human world and the pony world combined. The new spicies both tried to establish dominance, but no one could agree who would hold it.
A war broke out, a castrophic war. Many died, on both sides. The humans had superior technowlogy and were able to fire projectiles that moved too fast for even the best of unicorns to stop
The humans were winning, badly. Untill one day a young unicorn found a book that told of the human body and how it worked. The unicorns studied this book for months. Then in the next battle, the fight was finally even. The unicorns were able to stop organs from working in the human body, killing them instantly.
This bloody war would of made the two races extinct if it countinued. And it continued for a terrible 3 years. Millions died a month. And no victor was seen in the future.
Then one day. a young boy met an injured pony. The pony instantly felt hosility twords the boy, and was going to just kill him on the spot. Yet, the boy held something out to the pony, a medical pack. He had seen the pony earlier and smuggled it from the troops.
Reluctantly, the pony took the pack and saw its contents, and smiled at the boy. The two became close friends. The next day the boy woke up with a strange mark on his hand. It was the the cutie mark of the pony. The boy was 9 years old.
The boy snuck back to the humans, but was caught by the unicorns. The unicorns wanted to make an example of him. so they tied him up to a post. They were going to spear him through his arms and make him suffer. They were going to make his kind watch, as they were on an overpass looking over a current battle.
The pony the boy befrended ran in. He simple said "Stop" and everypony obeyed. For the pony was the ruler of their time, Princess Celestia. The alicorn told the boy to hold up his hand, and he obeyed. The ponies were marvled and shocked by the mark on his hand.
Five days later a peace treaty was issued, knowing that the two races could work togather. The humans took only 4 days to accept it, thanks to the boy. It soon later was discovered that the ponies could connect to the humans only at the age of 9. No one knew why, no one cared.
So a tradition started, ponies and children would grow up togather, and connect when they were 9. This new peace treaty was only a test run. and after putting it into place for 20 years, and it working, they kept it.
The ponies gave the humans their magic, through the marks they gave them. The humans shared their technowlogy with the humans. This was how it worked. How it always worked.